Q: Which equation correctly expresses Raoult’s Law for the vapor pressure of a solution containing a non-volatile solute?

$$P_\text{solution} = \chi_\text{solvent} P_\text{solvent}$$

Q: Which formula should be used to calculate the freezing point depression for an electrolyte solution?

$$\Delta T_f = i \times m \times K_f$$

Question 1

Why does spreading salt on icy roads help to melt the ice, even when the temperature is below 0°C?

Accepted Answer

Salt lowers the freezing point of water, making it possible for ice to melt at temperatures below 0°C.

Question 2

Which of the following best describes a colligative property?

Accepted Answer

A property that depends on the ratio of solute to solvent particles, not their identity.

Question 3

A solution is made by dissolving 2.0 mol of NaCl in 1.0 kg of water. Assuming complete dissociation, what is the van’t Hoff factor $$i$$ for NaCl, and how many moles of particles are present in the solution?

Accepted Answer

$$i = 2$$, 4.0 mol particles

Question 4

Which equation correctly expresses Raoult’s Law for the vapor pressure of a solution containing a non-volatile solute?

Accepted Answer

$$P_	ext{solution} = \chi_	ext{solvent} P_	ext{solvent}$$

Question 5

A 1.7-molal solution of ethylene glycol in ethanol is prepared. Given $$K_f$$ for ethanol is $$1.99\ ^\circ\mathrm{C}/m$$, what is the expected freezing point of the solution? (Ethanol’s normal freezing point is $$-114.1\ ^\circ\mathrm{C})$$

Accepted Answer

$$-117.5\ ^\circ\mathrm{C}$$

Question 6

Which of the following statements about ideal solutions is correct?

Accepted Answer

Ideal solutions have identical interactions between all components.

Question 7

A solution is made by mixing 0.50 mol of glucose (a non-electrolyte) with 1.0 kg of water. What is the expected boiling point elevation? ($$K_b$$ for water is $$0.512\ ^\circ\mathrm{C}/m)$$

Accepted Answer

$$0.256\ ^\circ\mathrm{C}$$

Question 8

Which solution will have the lowest freezing point, assuming complete dissociation?

Accepted Answer

0.50 m MgCl$$_2$$

Question 9

If the measured vapor pressure of a solution is lower than predicted by Raoult’s Law, what does this indicate about the intermolecular interactions?

Accepted Answer

Solute–solvent interactions are stronger than solvent–solvent interactions.

Question 10

A solution is prepared by mixing 0.52 mol of CS$$_2$$ and 0.42 mol of acetone at 35°C. The vapor pressures of pure CS$$_2$$ and acetone are 515 torr and 332 torr, respectively. What is the expected total vapor pressure of the solution, assuming ideal behavior?

Accepted Answer

$$433$$ torr

Question 11

Which of the following best explains why antifreeze (ethylene glycol) is added to car radiators in winter?

Accepted Answer

It lowers the freezing point of water, preventing it from freezing.

Question 12

Which formula should be used to calculate the freezing point depression for an electrolyte solution?

Accepted Answer

$$\Delta T_f = i 	imes m 	imes K_f$$

Question 13

A solution is made by dissolving 1.0 mol of a non-volatile, non-electrolyte solute in 1.0 mol of solvent. What is the mole fraction of the solvent?

Accepted Answer

$$0.50$$

Question 14

Which of the following is NOT a colligative property?

Accepted Answer

Color change of the solution

Question 15

Why is the measured van’t Hoff factor ($$i) $$for strong electrolytes often slightly less than the expected value?

Accepted Answer

Some ions pair up in solution, reducing the number of effective particles.

Question 16

Which of the following will increase the solubility of a gas in a liquid?

Accepted Answer

Increasing the pressure

Question 17

A 5.0 m glucose solution is prepared using 1 kg of water. What is the mole fraction of water in the solution? (Molar mass of water = 18 g/mol)

Accepted Answer

$$0.917$$

Question 18

Which of the following best describes the effect of adding a non-volatile solute to a solvent?

Accepted Answer

It decreases the vapor pressure of the solvent.

Question 19

Which of the following solutions will have the highest boiling point, assuming complete dissociation?

Accepted Answer

0.50 m MgCl$$_2$$

Colligative Properties and Solution Behavior (Ch. 13.6–13.7)

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